The invention is in the field of electrostatic spraying and relates particularly to space-charge controlled, low volume electrostatic spraying which is particularly suitable to agricultural environments but is useful in industrial and other environments as well.
Low volume electrostatic spraying has been used from time to time in agriculture to spray pesticides on crops. For example, Point U.S. Pat. No. 3,339,840 illustrates electrostatic spraying of tobacco crops with fungicide powder particles of an average diameter of about 10 to 30 microns which are charged by electrodes maintained at 150,000 volts. As opposed to the fairly wide use of such spraying in industry, its use in agriculture has been rare, for a variety of reasons including the hazard associated with the high voltages that have been needed to charge the spray particles and the uncontrollable changes in the open environment of agricultural spraying. For example, while it may be relatively easy and convenient in an industrial setting to properly shield electrically the area where electrostatic spraying takes place, so as to avoid the danger of an electric shock from the charging voltages that are typically of the order of 100,000 volts, it is generally not possible to do so in an agricultural setting, where spraying typically takes place from a moving vehicle exposed to atmospheric conditions and operated by personnel unskilled in using such high voltages. Moreover, while it may be possible to properly calibrate and optimize the many relevant parameters in an industrial setting, this may not be easy in an agricultural setting where parameters such as the humidity of the air and many other electrical characteristics of the environment can not be controlled. Still further, while it may be possible in an industrial environment to calculate or otherwise find an optimal value of certain parameters, such as charging voltage, distance between the spray nozzle and the sprayed object, etc., it has been often impractical or impossible to do so in an agricultural setting, where the relevant parameters change often and where there are few specialists in electrostatics.
There has been no practical and accepted system for electrostatic deposition in agriculture despite the great need for it and despite the great benefits that it would have brought about. For example, presently used, non-electrostatic spray application techniques are grossly inefficient; spray particle deposition efficiencies of less than 20% are typical in commercial crop growing. Moreover, the typical non-electrostatic spraying methods may use as much as 200 to 400 gallons of pesticide spray per acre, while it would be possible to use as little as 5 gallons or less per acre at the low volume spraying rates that are possible with electrostatic deposition. At such low volume spray rates there would be additional considerable savings of capital investment in storage and spraying equipment, savings in energy expenditures, and reduced danger to the environment, because of the considerably lower quantity of the substance needed for spraying a given area.
With this background, an object of the invention is to make it possible to widely use electrostatic deposition in agricultural environments, and to also make it possible to use simple and efficient electrostatic spraying in industrial and other environments as well.
In one embodiment of the invention, a substance is sprayed through a novel type electrostatic spray nozzle capable of operating efficiently at low charging voltages, of the order of a few thousand volts, e.g., 2 or 3,000 volts, as compared to the prior art where the typical operating voltages are of the order of 100,000 volts. All of the high voltage components of the new nozzle are enclosed, so as to make it safe for use in open environments such as in agriculture. The nozzle uses gas under pressure to form a stream of finely divided, electrostatically charged particles. A parameter related to the electrical space-charge density of the charged particles is monitored as the particles are directed for deposition on a calibration target simulating the actual target objects which are to be sprayed. The deposition of the charged particles on the calibration target is measured while the monitored parameter is varied, and the space-charge density corresponding to an optimal (maximum) deposition of the charged particles on the calibration target is chosen as a desirable one. Suitable controls are then set to maintain the space-charge density during actual spraying of target objects within a selected range corresponding to the selected optimum value of the monitored parameter which was found to give optimal deposition of particles on the calibration target.
It has been found that for any given environment there is an optimal space-charge density which results in optimal deposition of particles on any given target surface. The term "optimal" can be defined as "maximum" deposition for a given amount of material sprayed or as a "most uniform" deposition, or as some compromise between the overall amount of the particles deposited on the targets and the targets and the distribution of the deposition. Deviation from the optimal space-charge density in either direction means less than optimal deposition of particles on the target surfaces. The specific optimal space-charge density depends on so many different factors that it is difficult to calculate in many environments and is indeed impractical or impossible to calculate in an agricultural environment. Therefore, the monitoring, in accordance with the invention, of a parameter related to the space-charge density, while varying the space-charge density and depositing charged particles on a calibration target simulating the intended target objects, solves the optimization problem in a simple but effective manner. This approach makes it possible to use optimal electrostatic spraying in agricultural environments or any other environment where it is impossible or impractical to otherwise calculate or find the optimal space-charge density of the sprayed charged particles.
Thus, it has been found that there is a critical value for space-charge density of the sprayed particles and that departure therefrom results in less than optimal particle deposition on targets, with extreme departure from the critical space-charge level (either too high or too low) resulting in only marginal improvement in deposition efficiency over the spraying of particles which are not electrostatically charged. To establish reliability and increase efficiency in the electrostatic deposition of charged particles on plant surfaces or other targets, and to maximize particle deposition on such targets, it has been found, in accordance with the invention, highly desirable to sense the space-charge density of the charged particles, to find the optimum level thereof, and to automatically maintain this optimum level while depositing on the target objects. This is done, in accordance with the invention, such that the monitoring does not significantly disturb the charged particles and inherently compensates for changes in factors (such as ion concentration in the air, resistivity of the sprayed particles, inadvertent changes in spray flowrate or in fineness of particle atomization, etc.) which influence the sensed space-charge density and the cloud-breakdown problem near the sprayed targets.
There have been techniques in the prior art to monitor variables related to the space-charge density of electrostatically charged particles. For example, Ransburg et al. U.S. Pat. No. 2,509,277 discloses a system measuring the discharge current from an electrostatic spray gun used in an industrial environment and controlling the charging voltage so as to prevent arcing of the discharge current over to the grounded target or to other objects. This technique presupposes knowing what charging voltage would cause arcing before the control circuit can be calibrated accordingly, and also presupposes that there will be no substantial changes in the environment variables that affect arcing once the control circuit is calibrated. In general no such factors can be presupposed in agricultural or other uncontrolled environments. In contrast, the invention provides a simple and efficient way of determining exactly what the optimal space-charge density would be under any given conditions, without a previous knowledge of what it shoud be, and a way of maintaining such space-charge density for optimal deposition and not just to prevent arcing. As another example, Larsen et al. U.S. Pat. No. 2,767,359 shows a system in which the discharge voltage of a spray system is controlled so that the discharge current between the charging electrodes is constant. Again, this presupposes knowing what the discharge current should be in the first place, but does not find what would be an optimal space-charge density for optimal deposition of particles. As a still another example, Walberg U.S. Pat. No. 3,641,971 shows a system in which a control circuit is provided for cutting off the electrical power to a spray gun if the gun gets too close to a grounded object and thus causes a surge of the discharge current. This is only a protective device, and does not relate to finding an optimal value for the space-charge density of the sprayed charged particles.
In summary, the invention provides a significant improvement over the prior art and enables electrostatic spraying to be efficiently and safely used in many difficult environments, including agricultural environments. It uses a low volume spray nozzle, which is particularly safe to use in uncontrolled environments, to produce finely divided, electrostatically charged particles that may be liquid or solid. The charged particles are monitored to sense the value of a parameter related to their space-charge density. The particles are first deposited on a calibration target simulating the ultimate target object, and the space-charge density of the stream is varied while the degree and/or quality of the deposition on the test object is measured. The space-charge density corresponding to optimal deposition is thereafter maintained while the charged particles are being deposited on the target object.